First I'd say the quality and freshness of the meat. Hamburger ground that day preferably. It loses flavor pretty quickly. People will certainly debate about black angus beef, buffalo, etc. I don't care for them myself. Just a high quality, fresh meat from the butcher (ground that day).
While gas grills are considered upscale nowadays, you can't beat charcoal for grilling. On our gas grill, though, I like to use regular ground beef with lots of fat in it. (not that lean, wussy stuff) That way, a lot of grease drips down, causing flames and smoke, giving it that grilled taste. And regular ground beef holds together better than lean beef. I cook them fast so they get pretty well done on the outside, while keeping their moisture. Another way I keep them moist is by flipping as little as possible. Every time you flip you lose all that fat that bubbles up to the top. (You can also cover them inside the grill to keep the heat more even across the top.)
If I use seasoning, I only put it on the patty. I don't mix it all the way through. If you mix it in, you use 5 times as much to get half the flavor.
Good quality for other ingredients is essential, too. Kaiser rolls or some other favorite. Grill them for a bit to get them a little toasty to work well with a moist burger. Your personal fav pickles. Garden grown, real tomatoes. I personally like Duke's mayo and sea salt, too. Melt a good cheese while it's still on the grill.
Now I gotta run. The grill is calling me...